1. Field Of The Invention
The present invention relates to arc discharge lamps and more particularly to an arc discharge lamp which does not require a conventional ballast for operation.
2. Description Of Background Art
Typical high pressure discharge lamps comprise an arc tube which is hermetically sealed and has an electrode in each end. Within the arc tube is a quantity of mercury, a halide and a fill gas, for example. The arc tube is disposed within a conventional outer glass jacket with suitable lead-in wires.
It has long been known that electric discharge lamps require auxiliary circuits to ignite the discharge and to control the flow of current therethrough. This results from the fact that, first, the ignition voltage of such devices is higher than the operating voltage, and that, second, the operating voltage typically decreases with increasing current. Consequently if such a device is connected directly to the electric power lines, it either will not ignite, or if it does so, the current will increase rapidly to destructive levels.
As is well known, the usual type of arrangement for a high pressure discharge lamp includes a starter electrode, two principal electrodes, a switch and a ballast. The type of ballast depends on the wattage of the lamp and the voltage of the supply circuit. For example, the ballast may be a transformer type, a choke type, or a combination of transformer, inductance and capacitance.
The ignition process in prior art high pressure discharge lamps wherein the ballast is a choke coil is now described. When the switch is closed, an electric current flows through one of the principal electrodes and the starter electrode that is disposed adjacent to it. The discharge between the electrode and the starter electrode provides a plentiful supply of free electrons which aids the establishment of an arc between the two principal electrodes. The current through this arc is limited by the impedance of the choke. The heat from the arc vaporizes the mercury, raising the mercury pressure to the extent that the arc emits a brilliant light.
The behavior of an arc in a high pressure discharge lamp is quite different from the operation of an incandescent lamp. The high pressure discharge lamp is not a self-limiting device. An incandescent filament can only conduct a certain current for a given voltage because its resistance would increase if it gets hotter. However, as an arc becomes hotter, the resistance to the flow of electrons decreases. So the arc in a high pressure discharge lamp would "run away with itself" and destroy the lamp if it were not held back. This is the most important purpose of the ballast control circuit, whether it be a choke coil, a condenser, or a resistance.
As a result, the applications of electric arc discharge lamps have been limited to special fixtures equipped with such control circuits, called "ballasts". Arc discharge lamps cannot be used as direct substitutes for incandescent lamps in existing sockets. Although discharge lamp products have been and are presently on the market for such substitution, they incorporate the ballast circuit integrally in the housing or base of the lamp which greatly increases the cost of the product. Alternatively, the product may employ a "ballast adapter" which is inserted into the incandescent lamp socket, with a receptacle for connection to the discharge lamp. Such devices are cumbersome and costly. Consequently, even with these modifications, arc discharge lamps have not found significant application as incandescent lamp replacements. Therefore, the commercial and residential customers have been unable to enjoy to the fullest degree the significant energy saving that results from the lower power consumption of discharge lamps for the same light output, in comparison to incandescent lamps.
Despite the fact that this defect of discharge lamps has been known since the first large-scale commercial use of such lamps in the first decade of this century such as, for example, "COOPER-HEWITT" lamps, and despite the fact that many generations of lamp development engineers and scientists have addressed the problem, no practical ballastless discharge lamps have ever been provided.
It has also been known for many years that some electric discharge lamps have so called "positive volt-ampere characteristics", that is, over at least certain ranges of currents, the operating voltage of the discharge lamp increases with increasing current. An example of such a characteristic is set forth in the book entitled: The High Pressure Mercury Vapour Discharge by W. Elenbaas, North Holland Publishing Co., Amsterdam, 1951. As disclosed therein with respect to FIG. 19, E is the axial voltage gradient in the discharge, and P is the power input per unit length. Since operating voltage is equal to E times arc length plus electrode drop, and P equals axial electric field times current, from this curve can be constructed a curve of operating voltage versus discharge current, which illustrates that above a certain value of current, discharge operating voltage increases slowly with increasing current. It is described in the subject reference that the formula for axial voltage gradient is given by: ##EQU1## in which D is inside diameter of the discharge tube and M is mass of mercury vapor per unit length in mg/cm.
Despite the known existence of such a positive region to the Volt-ampere curve of a high-pressure mercury vapor discharge lamp, no practical ballastless lamps have ever been constructed on this principle. The problem is twofold: first, the high pressure discharge lamp cannot be ignited at the voltage drop at which it will operate on the positive branch of the V-I curve; and second, even if it could, the variation of discharge current within the normal range of tolerances of line voltages would be excessive and impractical. This can be shown by algebraic manipulation of Eq. 1, expressing P as equal to EI, and m in terms of the mercury vapor pressure in atmospheres, and focussing on the domain for P&gt;40 w/cm (i.e.&gt;&gt;9) EQU E=7.442 I.sup.1/5 (P.sub.HG).sup.7/10 /D.sup.2/5 Eq. 2
It can be seen that the axial voltage gradient increases as the 1/5 power of current. If the axial voltage gradient is fixed by the line voltage as (V.sub.L -V.sub.K)/L, solving for the current as a function of line voltage VL gives EQU I=[(V.sub.L -V.sub.K)/7.442 L].sup.5 (D.sup.2 /P.sub.HG.sup.7/2)Eq. 3
in which V.sub.K is electrode drop, and all other symbols have been defined. It is clear that for a known high pressure mercury discharge lamp, with L, D and mercury pressure fixed as design parameters, variation in line voltage must be accompanied by substantial variations in lamp current. Because of the fifth power dependence, for example, a 10% line voltage variation will be accompanied by a 50% variation in lamp current. Since +/-10% variations in line voltage are within the tolerance a high pressure mercury discharge lamp in this mode would lead to unacceptably large variations in power and light output under normal conditions of use.
For the two reasons outlined, therefore, no ballastless lamps have been developed making use of the positive V-I characteristics of the high pressure mercury discharge, known at least since the publication of the above referenced Elenbaas book, nearly 40 years ago.
It has been known to ignite a discharge in mercury vapor with so-called "mercury pool cathodes" by establishing a connection of liquid mercury within the device between the lead-in wires, so that current flows through the circuit. An interruption of the liquid path breaks that connection, and permits the establishment of the discharge in mercury vapor with a mercury pool cathode. The already-cited "COOPER-HEWITT" lamps were ignited in this way by tipping the lamp longitudinally to establish a continuous liquid mercury connection between electrodes, and subsequently restoring it to a horizontal position to break the connection. While this form of interruption of the liquid path has been known for many years, no change or modification has ever been employed for the purpose of igniting a discharge lamp to operate at normal line voltage without a ballast.
Accordingly a principal desirable object of the present invention is to provide a discharge lamp arc tube capable of ignition at ordinary line voltages without transformer step-up, and capable of subsequent operation at a predetermined design current from the same line voltage without ballast (series impedance) of any kind.
Another desirable object of the present invention is to provide a discharge lamp having a substantially higher efficacy than incandescent lamps.
Another desirable object of the present invention is to provide a discharge lamp which can successfully replace an incandescent lamp in the self-same socket yielding either equivalent light output at substantial reduction in power consumption, or substantially more light at equal power consumption.
A still further desirable object of the present invention is to provide formulae to facilitate the design of the discharge lamps of the present invention to provide predetermined performance specifications.
These and other desirable objects of the invention will in part appear hereinafter and will in part become apparent after consideration of the specification with reference to the accompanying drawings and the claims.